Some loans at the ill-fated Presbyterian Mutual Society (PMS) were granted on the back of an envelope, it has been claimed.
A special investigation claims rules were ignored to provide huge loans to non-members, with more than £5.5m given out in unapproved loans and £100m handed to seven property speculators.
The UTV Insight report – to be aired tonight – asks who was to blame for the collapse of PMS and looks at how it was allowed to gamble away millions of pounds of members' money.
Around 10,000 people with money in the PMS were affected when it went under in 2008 after it emerged that it was not covered by a UK government guarantee for savings deposits.
Reporter Chris Moore said five directors held responsibility for the failings of the PMS are unrepentant and see themselves as victims of the crisis.
One investor, Jennifer McNeill, said: "I didn't realise there were substantial figures being loaned to building companies and property developers. We just didn't know if we were ever going to see that money again."
Ombudsman Tom Frawley previously found Stormont's Department of Enterprise, Trade and Investment (DETI) failed to properly scrutinise the activities of PMS before it collapsed.
DETI Minister Arlene Foster said the department knows of "loans that were granted essentially on the back of an envelope".
She disputes the findings of the Assembly Ombudsman who says that there was maladministration in DETI that allowed many of the failings in the PMS to happen.
She said: "I don't take any of the blame in respect of the PMS failure... the directors were allowing the PMS to take in huge amounts of money, to lend out huge amounts of money without due diligence."
DETI took court proceedings against five PMS directors, resulting in their disqualifications.
When it collapsed, the PMS was not subject to regulation by the Financial Services Authority (FSA), yet it was investing in a manner that required FSA authorisation. If the society had been FSA-regulated, shareholders would have been protected by the statutory Financial Services Compensation Scheme after it was placed into administration. The issue was finally addressed in 2011 when ministers at Westminster and Stormont agreed a rescue package which meant shareholders with less than £20,000 in the society got all their money back, while those with more funds invested got the majority back.